Western Bumble Bee is a medium-sized bumble bee. It has a short head and a band of yellow hair across the thorax in front of the base of the wings. The tip of the abdomen is almost always white and the rest of the abdomen varies in colour. The subspecies found in the Northwest Territories (NWT) is the northern long-haired subspecies mckayi.

Western Bumble Bee occurs in western North America. The northern subspecies is found in northern British Columbia, Alaska, Yukon and the western mountains of the Northwest Territories. The mountains north of the Nahanni River drainage have not been surveyed for bumble bees, so the northern extent of the range is uncertain.

The Western Bumble Bee has an annual life cycle with one generation per year. In the spring, the Western Bumble Bee queen emerges from her overwintering site and searches for a suitable nest site in which to lay eggs. The eggs develop into female worker bees. The workers gather pollen and nectar for the colony and also take care of the nest and larvae. In late summer, reproductive bees (males and new queens) are produced. They leave the colony and mate. The mated queens enter hibernation; the old queen, workers and males are killed by frost. The new queens will emerge the following spring and establish new colonies.

Habitat

Western Bumble Bees can use a wide range of habitats, as long as flowers and nest sites are available. They eat pollen and nectar from various types of flowers. The colony usually nests underground in abandoned rodent burrows. The queens burrow into loose soil or rotting trees to overwinter.

Population size and trends

Recent surveys suggest that the northern subspecies mckayi of Western Bumble Bee is still common. However, the southern subspecies (Bombus occidentalis occidentalis) is experiencing a serious population decline. Because the reasons for the southern decline are unknown, there is cause for concern for the northern subspecies as well.

Threats and limiting factors

Western Bumble Bee has a high parasite load compared to other bumble bee species. Other threats that could become a concern if they occur in the NWT include the introduction of exotic bumble bee species for pollination and the use of pesticides.